Re: [PATCH v2] block: Reject misaligned write requests with BDRV_REQ_NO_FALLBACK

[Kevin Wolf <kwolf@redhat.com>]

Am 14.10.2019 um 10:15 hat Alberto Garcia geschrieben:
> The BDRV_REQ_NO_FALLBACK flag means that an operation should only be
> performed if it can be offloaded or otherwise performed efficiently.
> 
> However a misaligned write request requires a RMW so we should return
> an error and let the caller decide how to proceed.
> 
> This hits an assertion since commit c8bb23cbdb if the required
> alignment is larger than the cluster size:
> 
> qemu-img create -f qcow2 -o cluster_size=2k img.qcow2 4G
> qemu-io -c "open -o driver=qcow2,file.align=4k blkdebug::img.qcow2" \
>         -c 'write 0 512'
> qemu-io: block/io.c:1127: bdrv_driver_pwritev: Assertion `!(flags & BDRV_REQ_NO_FALLBACK)' failed.
> Aborted
> 
> The reason is that when writing to an unallocated cluster we try to
> skip the copy-on-write part and zeroize it using BDRV_REQ_NO_FALLBACK
> instead, resulting in a write request that is too small (2KB cluster
> size vs 4KB required alignment).
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>

Thanks, applied to the block branch.

Kevin